This invention relates to a coolant storage tank for a fuel cell system. In particular, it relates to a fuel cell water storage tank. The invention also relates to a fuel cell system including said coolant storage tank.
Conventional electrochemical fuel cells convert fuel and oxidant into electrical energy and a reaction product. A common type of electrochemical fuel cell comprises a membrane electrode assembly (MEA), which includes a polymeric ion (proton) transfer membrane between an anode and a cathode and gas diffusion structures. The fuel, such as hydrogen, and the oxidant, such as oxygen from air, are passed over respective sides of the MEA to generate electrical energy and water as the reaction product. A stack may be formed comprising a number of such fuel cells arranged with separate anode and cathode fluid flow paths. Such a stack is typically in the form of a block comprising numerous individual fuel cell plates held together by end plates at either end of the stack.
It is important that the polymeric ion transfer membrane remains hydrated for efficient operation. It is also important that the temperature of the stack is controlled. Thus, coolant water may be supplied to the stack for cooling and/or hydration. Accordingly a fuel cell system may include a water/coolant storage tank for storing water for hydration and/or cooling of the fuel cell stack, for example. If the fuel cell system is stored or operated in sub-zero conditions, the water in the fuel cell stack and water storage tank may freeze. The frozen water may cause blockages that hinder the supply of coolant or hydration water to the fuel cell stack. This is a particular problem on shut-down of the fuel cell system when the water in the water storage tank is no longer heated by its passage through the stack and may freeze completely. In such an event sufficient liquid water may not be available for hydration and/or cooling. This may prevent the fuel cell stack from being restarted or operating at full power until the frozen water has been thawed. It is known to provide a heater in the fuel cell system, which operates on stored energy, such as from a battery, and maintains the fuel cell system at above-zero temperatures to prevent freezing occurring. The battery power is, however, limited and the fuel cell system may experience freezing if the battery fails or becomes discharged.